The present patent application claims the right of priority under 35 U.S.C. xc2xa7119 (a)-(d) of German Patent Application No. 101 27 427.9, filed Jun. 6, 2001.
The invention relates to a method and an apparatus for the pneumatic conveying of fine bulk material, in particular glass fibers.
Background art: glass fibers are used in large quantities for manufacturing plastics materialxe2x80x94especially for manufacturing glass-fiber-reinforced plastics materials. In said case, a considerable outlay is necessary for handling the glass fibers, especially for transport and charging of the production installations. The space required for storage of the glass fiber bunches can be considerable.
Glass fibers are usually processed in the form of fiber bundles. The fiber bundles are composed of a number, e.g. 2000, parallel individual fibers having a typical length of around 4.5 mm.
Currently known conveying techniques for the intermittent charging/filling of production installations with cut glass fibers include elevator installations, industrial trucks such as fork lift trucks or crane installations.
Continuous conveying may be effected by means of, for example, bucket conveyors, continuous handling equipment, shaking or vibrating chutes or conveyor belts.
The drawback is that some of these conveying techniques are very elaborate and, because of their shape and spatial requirement, may be integrated only with difficulty into existing production facilities.
An alternative to these approaches may be offered by the use of pneumatic conveying installations. However, the operation of pneumatic conveying installations for conveying cut glass fibers presents various problems with regard to quality assurance and operational reliability.
With pneumatic airborne conveying, all of the bulk material particles or glass fibers in the form of fiber bundles move in approximate uniformity over the pipe cross section. In said case, particles having sizes above approximately 1 mm collide with one another and with the pipe walls, which leads to transverse movements and rotation of the particles. In the case of smaller particles, the transverse movements are caused by the turbulence of the air flow. The high conveying speedxe2x80x94the flow is generally turbulentxe2x80x94leads to considerable abrasion of the glass fiber bundles. The fiber bundles are undesirably partially broken up and ground into glass wadding and so may not be processed any further. A further undesirable outcome is that the conveying pipes may become clogged with glass fibers.
With pneumatic low-speed conveying, whereby the glass fibers are conveyed typically at speeds equal to or lower than 5 m/s, the comparatively lower speed leads to a partial deposit of glass fibers in the horizontal pipe. The resulting flow form is skein conveying. The deposited fiber fraction at the bottom of the pipe continues to be conveyed more or less quickly in the form of skeins. A further lowering of the conveying speed leads to a dune-like conveying of the deposited fibers and/or to conveying in plugs, which may fill the entire pipe cross section.
Given the use of pressure dispatchers (i.e., a pressure feed for charging glass fiber portions) for slow-speed conveying in the pipe system, the outlet region of the pressure dispatcher becomes clogged with the cut glass fibers.
In the case of conveying of the glass fibers by means of cellular wheel sluices as metering hoppers into the pipe system, in which the pneumatic conveying is effected, the cellular wheel sluice serving as the metering element may itself lead to the destruction of the cut glass fibers. This leads to blocking of the cellular wheel sluice by the glass fibers, which collect between rotor and stator of the cellular wheel.
Methods of effecting the low-speed conveying of bulk materials which are difficult to handle or require gentle handling, such as suction conveying using supplementary air valves or pressure conveying with supplementary lines, which carry pure gas and have an elastic inner pipe, alter the cut glass fibers too much at a high conveying speed on account of impact and friction between the glass fibers themselves and between the glass fibers and the conveying installation, e.g., a pipe. This leads to disintegration of the fiber bundles.
As a result of abrasion of the feed material the pneumatic conveying system is ground through. Known installations or materials for conveying systems are therefore operationally unreliable and too maintenance-intensive. There is also the possibility of the glass fibers being damaged during transport.
Thus, with the described conveying systems according to prior art it is not possible to achieve a satisfactory charging of the production plant with cut glass fibers, which is notable for high operational reliability, low operating costs (wear, handling) and gentle handling of material (quality assurance).
The object of the present invention is to provide an apparatus and a method of pneumatically conveying cut glass fibers, which is operationally highly reliable, low-maintenance and easy to operate and operates in a way that handles the material gently.
In accordance with the present invention, there is provided an apparatus comprising:
(i) a pneumatic conveying pipe (2) having upstream and downstream portions, said pneumatic conveying pipe having a first portion (2a) which is located upstream from a second portion (2b), and a discharge point (8) located further downstream from said second portion (2b);
(ii) a charging container (1) that is in reversibly closeable communication with a charging point (19) of said pneumatic conveying pipe (2), said charging point (19) being located further upstream from said first portion (2a) of said pneumatic conveying pipe (2);
(iii) a suction conveying device (3) which is in reversibly closeable gaseous communication with a suction point (22) of said pneumatic conveying pipe (2), said suction point (22) being located between said first (2a) and second (2b) portions of said pneumatic conveying pipe (2);
(iv) a pressure conveying device (16) that is in reversibly closeable gaseous communication with a portion of said pneumatic conveying pipe (2) at a point (34) that is further upstream from said charging point (19);
(v) a conveying gas supplementary line (4) having one of a closed end and a reversibly closeable end, said pressure conveying device (16) being in reversibly closeable gaseous communication with said conveying gas supplementary line (4); and
(vi) a plurality of shut-off devices (6) providing reversibly closeable gaseous communication from said conveying gas supplementary line (4) into said pneumatic conveying pipe (2).
In accordance with the present invention, there is also provided a method of pneumatically conveying a fine bulk material. The method comprises:
(a) providing the apparatus as described above;
(b) filling, in a first step, said first portion (2a) of said pneumatic conveying pipe (2) with a fine bulk feed material from said charging container (1) through said charging point (19) by means of said suction charging device (3) drawing said fine bulk feed material into said first portion (2a) of said pneumatic conveying pipe (2);
(c) transferring, in a second step, the fine bulk feed material from said first portion (2a) to said second portion (2b) of said pneumatic conveying pipe (2), by means of said pressure conveying device (16) which introduces conveying gas into said pneumatic conveying pipe (2); and
(d) conveying, in a third step, the fine bulk feed material from said second portion (2b) of said pneumatic conveying pipe (2) to said discharge point (8), by means of said pressure conveying device (16),
wherein each of said second portion (2b) of said pneumatic conveying pipe (2) and said conveying gas supplementary line (4) are provided with a constant quantity of conveying gas.
The features that characterize the present invention are pointed out with particularity in the claims, which are annexed to and form a part of this disclosure. These and other features of the invention, its operating advantages and the specific objects obtained by its use will be more fully understood from the following detailed description and the accompanying drawings in which preferred embodiments of the invention are illustrated and described.
Other than in the examples, or where otherwise indicated, all numbers or expressions, such a those expressing structural dimensions, etc, used in the specification and claims are to be understood as modified in all instances by the term xe2x80x9cabout.xe2x80x9d